Keto, low-fat, paleo, intermittent fasting, carnivore — head-to-head trials repeatedly find that when calories and protein are matched, the differences between diets are small. What actually predicts weight loss is energy balance (eating less than you burn), and what predicts keeping it off is adherence — whether you can actually live this way. Every diet that works does so mostly by making you eat less, usually by eliminating a whole category of food you were overeating.
This is why the diet debate is largely misframed. The question isn't 'which diet is metabolically superior' but 'which set of rules can you follow for years without feeling deprived.' Someone who hates meat will fail on carnivore no matter what the metabolic literature says. The best diet is the one that fits your food preferences, culture, schedule, and budget — because a diet you quit has an effectiveness of zero.
Stanford researcher Christopher Gardner ran the DIETFITS trial to settle the low-fat versus low-carb question properly: 609 adults, randomized, followed for a full year with real nutritional support rather than a handout. The result, published in JAMA in 2018, was that both groups lost roughly the same amount — about 5–6 kg on average — with no significant difference between them. Gardner also tested whether genetic markers or insulin response predicted who'd do better on which diet, a popular hypothesis at the time; they didn't. What the data showed instead was enormous variation within each group: some people lost 30 kg, others gained. The differences between individuals dwarfed the differences between diets. Earlier, Frank Sacks's 2009 NEJM trial comparing four different macronutrient combinations had reached the same conclusion — adherence, not composition, drove results.